986s new engine

[BoxsterSteve]

You are looking at a complete drivetrain and electrical system swap, not simply an engine transplant.
By the time you've tallied up all the costs of this sort of conversion, you could buy yourself a very nice 996, 997 or Cayman, put the key in and drive it.
Or you could spend a year or two busting knuckles and filling up your cuss box trying to do a very complicated driveline conversion in an at best 12 year old vehicle.
Just my $.02.